RE: Piston - valve kissing

[Chris <chris@domain.elided>]

"Rich Wagner" <drsavage@domain.elided> wrote in digest #733 :

> [a lot]
Hear, hear!

And this I also read:
> From: Ferdinando Di Matteo <ferdinando.di@domain.elided>
> Subject: RE: Piston - valve kissing

> I'm going to say this one more time, if the timing belt tensioner(s) are
> properly installed and adjusted by THE book, the engine turning backwards
> will NOT cause the timing belt to slip or jump teeth!  Any none believers
> out there?  Fred Di Matteo  AROC Technical Advisor from Fort Myers, Florida

I totally agree with both of you.

Timing belts don't slip, unless, -ah, there's always a unless- they're
not tightened properly.
Alfa wouldn't want the risk, I think, of having timing belts skip unless
they had a lot of engines laying around which they couldn't get rid of.

Let me explain:
When shutting down an engine you perform one or more of these things:
    1. No more power to coil (bobine)
    2. Close valve in carb (aka solenoid)
    3. Turn off the ECU (brains)
    4. Turn off the fuelpump (diesels, injected)

Which should be enough to kill a running engine.
Weren't it that _EVERY_ (otto)engine, no matter what fuel they burn
or what method of fuel intake they use, _will_ turn backwards once in
a while. Although injected engines suffer less from this nuisance, they
do also. The reason for this is that, when you're driving, not all fuel
gets burned and is pushed in the carter. The carter ventilation on its
turn makes sure you burn this "overflow" of fuel. Now when shutting down
the engine (see above), there's always some fuel to burn...

Consider a 1 cylinder motor.
Shut it down, and imagine the compression stroke....
There was not enough momentum to get it through this, so it turns the
other way sucking from the carter and exhaust.... lots of gasses there
happy to explode if heated enough. Your engine, plugs, etc. being 
adequately warm will ignite this mixture, which causes your engine to
turn backwards (this fenomenon is also called "pinging", not to be
confused with the ordinary "pinging" you get with too low octane fuels)
On the next stroke, more exhaust gases and carter "fuel" is brought into
the cylinder which happily ignites, etc., etc... old engine with lots of
"coal" deposits helps this scenario.

With 6 cylinders there's some more chance of this happening.
(now don't go shouting "Yes, but there are other cylinders opressing
this movement", I know I'm right.)

You cannot make me believe the Milano engineers haven't taken this into
account. (although they have funny thoughts on electronics)

And then there's this:
The "tensioner" actually is a "DE-tensioner".
ie. it makes sure your timing belt is nice and tight (if properly
adjusted) on cold starts. Then when your engine warms, it actually
releases tension on your timing belt, and here's where "Rich Wagner"'s
story steps in.

Also, the difference between the original -oilfed- and mechanical
tensioner -besides leaking oil- is: the latter won't give the proper
tension on a warm engine.(?!)

So, maybe all you "belt-skip" victims had the original replaced with
the mechanical variant, and _then_ turned the engine ACW which caused
the slip?
(I'm _NOT_ contradicting my above story. With the original tensioner,
as soon as there's no oil pressure (engine turning backwards) , it'd
tension the belt again (spring loaded as it is))

Just my L200, 

stepping down from the soap box... NEXT!

    Chris Piepers
    The Netherlands

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